1. Field of the Invention
The present invention relates to an optical fiber of which the core is co-doped with at least a rare earth element, phosphorous, and aluminum, and a manufacturing method for an optical fiber preform that is suitable for the manufacture thereof.
2. Description of the Related Art
As a factor that interferes with the performance of a fiber amplifier and a fiber laser, a phenomenon called a non-linear optical effect is known. This phenomenon occurs when the power of light that propagates through the core of an optical fiber is fairly high, and gives rise to problems such as wavelength conversion of light. For example, in a high-output fiber laser of which the output light power exceeds several tens of Watts, the occurrence of stimulated Raman scattering, which is a type of non-linear optical effect, generates light with a wavelength that is somewhat longer than the wavelength of the output light. As a result of this light being amplified, the spectrum width of the output light is disadvantageously widened. Therefore, it is important to design an optical fiber so as to inhibit the generation of non-linear optical effects represented by stimulated Raman scattering.
In general, a non-linear constant γ of an optical fiber is given by the following Equation (1).γ=(2π/λ)×(n2/Aeff)  (1)
In Equation (1), λ is the wavelength, n2 is the non-linear refractive index, and Aeff is the effective cross-sectional area. As is clear from Equation (1), it is effective to increase Aeff. in order to inhibit the generation of the non-linear optical effects
On the other hand, the following is known with regard to the relationship between the distribution of the relative refractive index difference of the optical fiber and the Aeff. FIG. 6 to FIG. 8 show the distribution profile of the relative refractive index difference in the radial cross-section of the core of an optical fiber. In the figures, the horizontal axis denotes the radial position, and the vertical axis denotes the relative refractive index difference (%). The size of the Aeff is in the relationship of “optical fiber of FIG. 6>optical fiber of FIG. 7>optical fiber of FIG. 8” given that the core diameter and the relative refractive index difference are the same throughout (refer to Proceedings of the SPIE, Vol. 5335, pp. 132-139 (2004)). As shown in FIG. 6 and FIG. 7, as the distribution profile of the relative refractive index difference becomes closer to a rectangle, the electric-field distribution of light that propagates through the core of the optical fiber becomes broader, and Aeff thus becomes greater. On the other hand, as shown in FIG. 8, as the distribution profile of the relative refractive index difference becomes closer to a bell shape, Aeff becomes smaller. Here, the “relative refractive index difference (Δ)” is expressed by Equation (2) given below. In Equation (2), ncore denotes the refractive index of the core, while nclad denotes the refractive index of the cladding.Δ(%)=(ncore−nclad)/ncore×100  (2)
For example, as a method of manufacturing an optical fiber preform by adding ytterbium (Yb) as a rare earth element, a method is known in which glass particles that consist of silicon dioxide (SiO2) are deposited in a silica tube by the modified chemical vapor deposition method, and Yb and Al are added by the solution doping method using an aqueous solution that includes Yb and aluminum (Al). The distribution profile of the relative refractive index difference of the optical fiber preform that is manufactured by the method is normally a bell shape as shown in FIG. 8. This is because the bulk density of the glass particles varies in the radial direction if the glass particles are deposited by the modified chemical vapor deposition method. In the modified chemical vapor deposition method, since the silica tube is heated from the outer wall surface, as the position becomes closer to the inner wall surface of the silica tube, the temperature becomes higher and the bulk density of the glass particles becomes greater. On the other hand, as the position becomes closer to the center of the silica tube, the temperature becomes lower and the bulk density becomes lower. In the solution doping method, the lower the bulk density of the glass particles (i.e., the higher the porosity), the higher the doping amount of the dopant. Accordingly, when Yb and Al are added using the solution doping method, the distribution profile of the relative refractive index difference becomes bell shaped as shown in FIG. 8.
As mentioned above, as a method of manufacturing a rare earth doped optical fiber preform, a method is known in which glass particles are deposited in a silica tube by the modified chemical vapor deposition method, and a rare earth element is added to the glass particles by the solution doping method. In addition, a method is known in which aluminum is co-doped in order to inhibit clustering of the rare earth ions. As methods of aluminum co-doping, a method of using the solution doping method (refer to Japanese Patent No. 2931026), and a method of using the modified chemical vapor deposition method in which AlCl3 gas is allowed to flow into a silica tube when depositing glass particles on the silica tube (refer to Japanese Unexamined Patent Application, First Publication No. 2003-137542).
On the other hand, as a phosphorous (P) co-doping method, a method of using the modified chemical vapor deposition method is disclosed in which POCl3 gas is allowed to flow into a silica tube when depositing glass particles on the silica tube (refer to Japanese Examined Patent Application No. H04-059254).
As mentioned above, the methods of adding a rare earth element, Al and P, when manufacturing an optical fiber preform, have already been disclosed. Then, after adding the desired dopant using both the modified chemical vapor deposition method and the solution doping method as described above, it is necessary to heat the silica tube to sinter the glass particles. However, the relationship between the steps of adding each dopant and the characteristics of the optical fiber preform and optical fiber when co-doping a rare earth element, Al and P is not known in detail. As matters stand, an optimal manufacturing method of an optical fiber that can suppress the generation of non-linear optical effects when used in a fiber amplifier and fiber laser has not been disclosed.
The present invention was conceived in view of the above circumstances, and the object thereof is to provide an optical fiber in which the concentration of a rare earth element is easily controlled, the effective cross-sectional area (Aeff) is large and non-linear optical effects when used in a fiber amplifier and fiber laser can be effectively suppressed, and a manufacturing method of an optical fiber preform that is suitable for manufacturing the optical fiber.